Phenotypic differences in terrestrial frog embryos: effect of water potential and phase

The terrestrial embryos of many amphibians obtain water in two ways; in a liquid phase from the substrate on which eggs are deposited, and in a vapour phase from the surrounding atmosphere. We tested whether the mode of water flux (liquid or vapour) affected the morphology and metabolic traits of the terrestrial Victorian smooth froglet (Geocrinia victoriana) embryos by incubating eggs both with a liquid water source and at a range of vapour water potentials. We found that embryos incubated with a liquid water source (psi(pi)=0 kPa) were better hydrated than embryos incubated with a vapour water source (psi(v)=0 kPa), and grew to a larger size. Eggs incubated in atmospheres with lower psi(v) values showed significant declines in mass and in the thickness of the jelly capsule, while embryos primarily showed reductions in dry mass, total length, tail length and fin height. The most significant deviations from control (psi(v)=0 kPa) values were observed when the psi(v) of the incubation media was less than the osmotic water potential (psi(pi)) of the embryonic interstitial fluid (approximately -425 kPa). Despite the caveat that a psi(v) of 0 kPa is probably difficult to achieve under our experimental conditions, the findings indicate the importance for eggs under natural conditions of contacting liquid water in the nesting substrate to allow swelling of the capsule.